Leeds Ecosystem, Atmosphere & Forest (LEAF) Centre

Climate

In the UK, urban temperatures are typically 1-2oC higher than the surrounding rural areas (Watkins et al., 2002; Jones & Lister, 2009). This urban heat island (UHI) effect occurs because the materials used to build towns and cities absorb more of the sun’s energy than the natural surfaces they replaced.

The UHI effect makes people living in urban areas particularly vulnerable to heat waves, for example there was an estimated 42% increase in mortality in London during the heatwave that affected Europe in August 2003 (Johnson et al., 2005).

Urban green spaces reduce the UHI effect by providing shade and by cooling the air through the process of evapotranspiration. During evapotranspiration, the sun’s energy is used to transfer water from the leaves of plants into the atmosphere (Grimmond & Oke, 1991).

Urban green spaces are on average around 1oC cooler, during both the day and night time, than built-up regions in the same town or city (Bowler et al., 2010), and this cooling effect can extend beyond the green space itself, into the surrounding urban areas (Yu & Hien, 2006). Large parks containing many trees with wide canopies, and minimal paving, reduce the urban heat island effect the most (Potchter et al., 2006; Chang et al., 2007; Bowler et al., 2010). During the summer this may reduce the need for air conditioning, and associated energy use, in nearby buildings (McHale et al., 2007).

The amount of carbon dioxide (CO2) in the atmosphere has increased by more than 40% since humans began industrialising, resulting in a gradual warming of the planet over the past century (IPCC, 2013). Trees and plants take carbon dioxide from the atmosphere and around half of it is stored in their branches and roots, with large amounts of carbon also stored by the surrounding soils.

This process is known as carbon sequestration and, as long as the vegetation is preserved, results in an overall reduction of atmospheric carbon dioxide concentrations. However, the decomposition of dead trees and plants returns carbon dioxide to the atmosphere. Understanding the carbon balance of any green space therefore requires an analysis of the relative amounts of sequestration and decomposition, in addition to any maintenance related greenhouse gas emissions (e.g., through mowing, irrigation and the use of fertiliser).

Overall, urban green spaces take in more carbon than they return to the atmosphere (Nowak et al., 2002; 2013) but their design and maintenance play a crucial role in determining how much carbon they will store. For example, a “forest-like” green space with many trees and native vegetation ground cover maximises carbon sequestration over a “park-like” design with fewer trees and frequently mown grass (Strohbach et al., 2012). As well as creating new green space, looking after existing mature trees is particularly important because they continue to sequester and store large amounts of carbon (Stephenson et al., 2014).